I am currently looking to buy a 3D printer. I've done some research about which technologies and materials they use. I've seen some models seems good so far but my concern is 'Are they mechanically solid?'. I don't want it to become a scrap after some usage. Since there are a lot of enthusiasts here that use 3D printer regularly, I would like to get use of your experience with 3D printers and their producer brands.
I am not buying it for hobby. I'll mostly use it to make cases for circuit boards.
Not surprisingly, your question is a difficult one to pin down in a precise manner. I'm going to pull one line from your post as the focus for my answer.
I would like to get use of your experience with 3D printers and their producer brands
I would suggest that you isolate a few models (or more) that hold your interest. A good example of a popular, quality printer is the Prusa i3 MK2s, although some would consider it to be expensive. It's available in kit form, as well as fully assembled, another topic entirely.
Find the forums specific to the printer models you've selected and read as many posts as you can. Look for user postings describing problems and suggested solutions. Look for user postings describing modifications which implies a problem that had to be solved by the user or community before the printer performed satisfactorily.
I've assisted in the building of a Prusa i3 MK2s recently and am currently assisting in a home-spun design based on a number of other generic printers. The owner of this model is incorporating as many modifications as he can find. That tells me that this model, although not yet completed, is likely to be a handful to tune and operate to his/my satisfaction.
On the flip side, everything I've read about the Prusa told me that for the public library for which it was destined, it would be the lowest level of trouble for any model I researched. I was asked to make a recommendation and performed the suggested research via all the forums I could find.
The above sounds like a direct recommendation, and it could be, but you have not provided too many specifics. The Prusa i3 MK2s (genuine, not clone) would likely print well for your circuit board cases, but there certainly would be other models to do the same job well enough.
Watch out for print bed size, based on your board size requirements. Heated beds usually mean more money, but greater variety of materials can be used as a result.
If you plan to limit your use specifically to circuit board cases, you may not have to be concerned about "expandability" from a feature standpoint. Single extruder models are fine for almost all 3D printing, but it's handy sometimes to have dual nozzle capacity. That's another plus for the Prusa i3 MK2s with an add-on kit, but not a big deal for case building, I think.
Look in the various forums that provide general support for 3D printing (such as this one) for other posts from people who use Brand X 3D printer and have had a problem with, for example, bed adhesion, filament quality, layer shifts, nozzle clogging, just about anything.
As with many things, high quality usually means higher price. The library Prusa has been zero problems for me to support on a volunteer basis, hence my recommendation. The makerspace also enjoyed the building process by buying the kit, saving US$200 in the process and learning more about the printer.
Keep an eye out for users who have experienced broken components or typically troublesome components. You would not want to buy a printer model that ships with a junky hot-end assembly if the suggested modification is to upgrade to an E3D V6 to make it work properly. A user who posts about a continuing belt breakage (rare!) could have a user-induced problem or a printer design problem, which means you'll want to cull out some of the information you see.
Good luck, it's a tough decision, to be sure.